1. Field of the Invention
The invention relates to a predistortion circuit for providing a linear output from a transmission device which has an output distorted from its input due to inherent nonlinearly, more particularly to a predistortion circuit that has a simple construction.
2. Description of the Related Art
It is well known to modulate the analog intensity of an optical source, such as a semiconductor laser, using an electric signal, in order to transmit analog signals, such as sound or video signals, on optical fibers. Distortion inherent in analog transmitters prevents an electrical modulation signal from being converted linearly to an optical signal, and instead distorts the signal. This is especially problematic with a multi-channel video transmission system, such as cable television. Therefore, a predistortion circuit is used to reduce the distortion inherent in nonlinear devices.
Referring to FIG. 1, optical power intensity of a nonlinear semiconductor laser in relation to current flowing therethrough is determined according to the following Equation 1: EQU P.sub.out =a.sub.1 I+a.sub.2 I.sup.2 +a.sub.3 I.sup.3 + (Equation 1)
where "P.sub.out " is the optical power intensity of the semiconductor laser, "I" is the current flowing through the semiconductor laser, and "a.sub.1,a.sub.2,a.sub.3, . . . " are coefficients of nonlinear Taylor expansion.
Referring to FIG. 2, an input signal is split into a main path and a secondary path (P.sub.1, P.sub.2). "I.sub.1 ", which is the current flowing through the main path (P.sub.1), and "I.sub.2 ", which is the current flowing through the main path (P.sub.2), in relation to the signal voltage (V) can be determined according to the following Equations 2 and 3: EQU I.sub.1 =b.sub.1 V+b.sub.2 V.sup.2 +b.sub.3 V.sup.3 + (Equation 2) EQU I.sub.2 =c.sub.1 V+c.sub.2 V.sup.2 +c.sub.3 V.sup.3 + (Equation 3)
If the relationship of "P.sub.out " to the signal voltage (V) is in accordance with the following Equation 4: EQU P.sub.out =k.sub.1 V+k.sub.2 V.sup.2 +k.sub.3 V.sup.3 + (Equation 4)
By introducing I=I.sub.1 =I.sub.2 into Equation 1 and by comparing Equation 4, we can obtain the following: EQU k.sub.1 =a.sub.1 (b.sub.1 +c.sub.1) (Equation 5.1) EQU k.sub.2 =a.sub.2 (b.sub.2 +c.sub.2)+a.sub.2 (b.sub.1.sup.2 +c.sub.1.sup.2 +2b.sub.1 c.sub.1) (Equation 5.2) EQU k.sub.3 =a.sub.3 (b.sub.3 +c.sub.3)+a.sub.2 (2b.sub.1 b.sub.2 +2c.sub.1 c.sub.2 +b.sub.1 c.sub.2 +b.sub.2 c.sub.1)+a.sub.3 (b.sub.1.sup.3 +c.sub.1.sup.3 +3b.sub.1.sup.2 c.sub.1 +3b.sub.1 c.sup.2.sub.1) (Equation 5.3)
If the coefficients (C.sub.I, I=1, 2, 3, . . . )of the nonlinear current (I.sub.2) flowing through the secondary path (P.sub.2) can be adjusted such that EQU k.sub.2 =k.sub.3 =. . . =k.sub.n =. . . =0 (Equation 6)
P.sub.out =k.sub.1 V in Equation 4, which is the ideal aim.
Conventional predistortion circuits are disclosed in U.S. Pat. Nos. 4,992,754, 5,132,639, 5,252,930, and 5,424,680.
The following are some of the drawbacks of the predistortion circuits disclosed in the aforesaid U.S. Patents:
1. In U.S. Pat. Nos. 4,992,754 and 5,132,639, two predistortion generators are needed to provide second-order and third-order predistortion compensation, thereby resulting in greater complexity and higher costs.
2. In U.S. Pat. No. 5,252,930, only second-order predistortion compensation can be provided such that the distortion compensating effect is inferior.